1. Field of the Invention
This invention relates to novel liquid-crystalline substances exhibiting a liquid-crystalline phase within a broad temperature range and also having a positive dielectric anisotropy, and liquid-crystalline compositions containing the same.
2. Description of the Prior Art
Display elements using liquid crystals have come to be broadly used for watches, desk calculators, etc. Such liquid-crystalline display elements utilize properties of the optical anisotropy and dielectric anisotropy of liquid-crystalline substances, and the liquid-crystalline phase includes nematic liquid-crystalline phase, smectic liquid-crystalline phase and cholesteric liquid-crystalline phase. However, display elements utilizing nematic liquid crystals among the above-mentioned phases have been practically used most broadly. Further the display mode of such display elements includes TN type (twisted nematic type), DS type (dynamic scattering type), guest-host type, DAP type, etc. and properties required for the liquid-crystalline substances used for the respective types are different. At any rate, however, it is preferred that the liquid-crystalline substances used for these display elements exhibit a liquid-crystalline phase within as broad a temperature range as possible in the natural world. At present, however, no single compound which satisfies by itself such conditions is present, and it is the present status that substances which are endurable to practical use for the present have been obtained by blending several kinds of liquid-crystalline compounds or non-liquid-crystalline compounds. Further, these substances should, of course, be stable to moisture, light, heat, air, etc. and also it is preferred for them that the threshold voltage and saturation voltage required for driving such display elements be as low as possible and their viscosities be as low as possible in order to make the response speed higher.
As compounds for satisfying such requirements, such compounds as expressed by the formulas ##STR5## (wherein R.sub.1 represents an alkyl group; X represents ##STR6## and R.sub.2 represents an alkyl group, an alkoxy group or CN) disclosed in U.S. Pat. No. 4,229,315 have been well known and practically used. However, requirements for liquid-crystalline display elements have become severer, and in order to satisfy the requirements, liquid-crystalline compositions which exhibit a liquid-crystalline phase over the range from a lower temperature to a higher temperature have become necessary.
Now, in order to broaden the liquid-crystalline temperature range toward higher temperatures, it is necessary to use liquid-crystalline substances having a higher melting point as a component, but such liquid-crystalline substances having a higher melting point generally have a higher viscosity and hence liquid-crystalline compositions containing these also have a higher viscosity; thus the response speed, particularly that at lower temperatures, of liquid-crystalline display elements which are usable up to higher temperatures such as 80.degree. C. have been liable to be notably retarded. The present inventors have made strenuous studies for obtaining liquid-crystalline substances satisfying the above-mentioned requirements, and have found unchanged or not increased so much in spite of their .DELTA..epsilon. values being reduced down to small values and also their viscosities are not elevated so much. Thus they are very useful compounds. Further they have superior stability to moisture, heat, light, air, etc.
The compounds of the formula (I) are concretely classified into those expressed by the following 17 formulas (II).about.(XVIII) (wherein each R has the same definition as in the formula (I)): ##STR7## (wherein Z represents F, Cl or CN; this applies to the following formulas (III) and (IV)) ##STR8## (wherein Y and Z each represent F or Cl; this applies to the following formulas (VI).about.(XII)) ##STR9## liquid-crystalline substance which exhibit a liquid-crystalline phase up to higher temperatures and nevertheless have a lower viscosity.